Asteroids are stony or metallic lumps of matter that
orbit the Sun in the vast volume of space between the orbits of Mars and
Jupiter. Made of stone or metal, the known asteroids range in size from
Ceres, the largest known asteroid at 923km in diameter to Aten, 1km in
diameter, and smaller. The total mass of all the known asteroids is less
than that of the Moon. Even the largest asteroids on the scale of this
model are so tiny as to be invisible. There are over 7000 known asteroids,
with hundreds more discovered each year.

Photos courtesy NASA.

Asteroid Gaspra's
Best Face The spacecraft Galileo encountered and photographed two asteroids
during its journey to Jupiter. This is the best color image of the asteroid
Gaspra. Color variations have been added. The illuminated portion of the
asteroid is about 11 miles long.

Ida and Dactyl: Asteroid
and Moon The second asteroid that the spacecraft Galileo photographed,
called Ida, was discovered to have a moon. It appears as a small dot to
the right of Ida in this picture. The tiny moon, named Dactyl, is about
one mile across, while the potato shaped Ida measures about 36 miles long
and 14 miles wide.